The Molecular Virology and Neurogenetics Section develops lentiviral vectors for the delivery of transgenes to the nervous system with current focus on the transduction of motor neurons by either retrograde transport of the vector after intramuscular injection or by direct injection into the spinal cord. Depending on the success and the efficacy of these routes of transgene delivery, select animal models for motor neuron diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) will be used to evaluate the efficacy of gene therapy. These vectors will also be useful for basic studies of motor neuron function. HIV-1-based vectors were isolated, each pseudotyped with a different glycoprotein derived from select rhabdoviruses. Two glycoproteins were identified that allowed purification of high vector titers and enabled the vectors to transduce muscle-identified motor neurons and muscle fibers after intramuscular injection in mice. Without optimization, the efficacy of motor neuron transduction was significant for each of the four injected muscle types. The success of this intramuscular route of transgene delivery represents a first for a HIV-1-based vector. This pseudotype vector could potentially be used therapeutically for motor neuron and muscle diseases, including but not limited to ALS and SMA. It may also allow transduction of other cells of the nervous system. HIV-1-based vectors have recently been used in clinical trials in AIDS patients and the potential safe use of HIV-1-based vectors for gene therapy in the human nervous system is encouraging. To complement the efficiency of transgene delivery by the vector: First, a minimal size promoter was identified, which retains neuron-specific gene expression. Second, from this minimal size promoter two novel chimeric neuron-specific promoters were constructed, which show increased activity in neurons either after neuronal depolarization or after neuronal injury. Towards gene therapy for neurological diseases, several vector genomes have been assembled that encode transgenes to either replace or to complement an endogenous defective gene or to express a silencing or neuroprotective transgene. These transgenes can now be packaged into the novel vector pseudotypes. In the future, the efficacy of these vectors will be evaluated in animal models for SMA and ALS, or in a focal epilepsy model or other models of neurological disease. The targeted transduction of a specific cell type within a mixed cell population represents an enormous challenge, especially when the target cell is in the minority. Pseudotyped HIV-1-based vectors were developed, which were able to target and specifically transduce cells that express the HIV-1 envelope (Env) protein on their surface and serve as a model for HIV-1 infected cells. To specifically target these Env+ cells, various chimeric CD4 and CXCR4 proteins were inserted into the vector envelopes to function as HIV-1 receptor and co-receptor. The transduction of HIV-1 expressing Env+ cells by HIV(CD4/CXCR4) vectors was very efficient in homogenous Env+ cell populations in vitro. However, in defined, mixed Env+/Env- cell populations, which better mimic in vivo conditions, the targeting of Env+ cells was unexpectedly very inefficient. Random and strong cell adhesion by the vector particles was detected with diverse cell types, which effectively competed with the targeted transduction of Env+ cells. These results have important implications for the efficacy of cell targeting by enveloped vectors in vivo.